Survival and energetic costs of repeated cold exposure in the Antarctic midge, Belgica antarctica

In this study, we examined the effects of repeated cold exposure (RCE) on the survival, energy content, and stress protein expression in larvae of the Antarctic midge, Belgica antarctica (Diptera: Chironomidae). Additionally, we compared results between larvae that were frozen at -5°C in the presence of water during RCE and those that were supercooled at -5°C in a dry environment. While over 95% of larvae survived a single 12 h bout of freezing at -5°C, after 5 cycles of RCE survival of frozen larvae dropped below 70%. Meanwhile, the survival of control and supercooled larvae was unchanged, remaining around 90% for the duration of the study. At the tissue level, frozen larvae had higher rates of cell mortality in the midgut than control and supercooled larvae. Furthermore, larvae that were frozen during RCE experienced a dramatic reduction in energy reserves; after 5 cycles, compared to supercooled larvae, frozen larvae had 25% less lipid, 30% less glycogen, and nearly 40% less trehalose. Finally, larvae that were frozen during RCE had higher expression of hsp70 than those that were supercooled, indicating a higher degree of protein damage in the frozen group. Results were similar between larvae that had accumulated 60 h of freezing at -5°C over 5 cycles of RCE and those that were frozen continuously for 60 h, suggesting that the total time spent frozen determines the physiological response. Our results suggest that it is preferable for larvae to remain unfrozen during RCE.